Power control device of a power network of an electrochemical coating facility

ABSTRACT

A power control device ( 1 ) of a power network ( 2 ), which has a number of anodes ( 5 ) and a number of cathodes ( 3 ), of an electrochemical coating facility is disclosed, having a plurality of control modules ( 6 ), each control module ( 6 ) being configured to calculate and control a local current flow having a predetermined quantity as a function of the location and as a function of the time between an anode ( 5 ) and a cathode ( 3 ) of the power network ( 2 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of International Application No. PCT/EP2008/061977 filed Sep. 10, 2008, which designates the United States of America, and claims priority to German Application No. 10 2007 045 149.2 filed Sep. 20, 2007, the contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The invention relates to a device for the power control of a power supply system of an electrochemical coating facility.

BACKGROUND

In an electrochemical coating facility, workpieces are coated locally or globally in respect of their area with material layers by means of a potential difference being produced between the workpiece to be coated and a medium in which the coating material is dissolved, and/or between the medium and external electrical conductors, which potential difference leads indirectly to condensation of the coating material on the workpiece. In addition to the change in the state of matter of the coating material, the latter can also change chemically in the course of the condensation process on the workpiece. The medium can be in a liquid, gaseous or plasmatic state of matter and can be the coating material itself, or can be a solvent or transport medium which contains the coating material.

Known electrochemical coating methods comprise plasma coating methods, for example, in which, in general, highly dilute gases are ionized by high-field excitations and thereby put into a plasmatic state of matter. As a result of chemical reactions in the plasma, the reaction products can then deposit (sputtering) on a substrate—in particular on a workpiece to be coated. A further important subgroup of the electrochemical coating methods is the electrolytic coating methods, in which ion diffusions are induced in an electrically dissociable medium by means of an externally applied electrical potential, which can lead indirectly to material deposition on a workpiece introduced into the medium. In this way, in electroplating technology, for example, workpieces are coated with metals by means of melts or solutions of metal salts being electrolytically separated. In this case, a—generally metallic—workpiece is conductively connected to an electrode, in particular a cathode, and an external electrical potential is applied between the electrode and the corresponding other electrode, in particular the anode. The positively charged metal ions (cations) in the metal salt melt or solution migrate to the cathode, are electrically neutralized upon contact with the workpiece and deposit as metal atoms on the workpiece. Coating methods in which workpieces are introduced into a usually liquid medium for the purpose of coating are also known as dipping bath coating methods.

In the case of an electrolytic dipping bath coating, the layer application per unit time onto the workpiece to be coated is a function of a number of parameters, which include, in particular, the applied electrical potential, the time and the thickness of the material layer already applied to the workpiece. Firstly, there is a decrease in the ion concentration in the medium given a constant potential in the course of coating with time, and therefore, given chemical properties that are otherwise kept constant, in particular the ion concentration of the bath, there is also a decrease in the current intensity of the ion current migrating to the cathode and, consequently, the layer application per unit time. This effect can be intensified if the material layer already applied has an insulating effect, which is in turn dependent on the conductivity of the cathode material, the time-dependent conductivity of the medium with the ions and on the conductivity of the layer material and also on the ratios of these conductivities. When all of the influencing parameters are taken into account, the layer application per unit time generally decreases overall given a constant electric potential, with the result that, in order to form a temporally linear layer thickness increase onto the workpiece, the electric potential has to be increased continuously with the residence time.

In the case of commercial dipping bath coating facilities designed for the coating of large workpieces, for example for the coating of vehicle bodies, power units are generally available for the DC voltage feeding the dipping bath, with which power units, for technical reasons, only a few, substantially constant potential values can be set. These potential values are also referred to as voltage levels. Furthermore, a change between the voltage levels in the dipping bath in the course of a coating process unfavorably causes discontinuities in the layer application; in particular, in the event of a changeover from one voltage level to the next higher voltage level, current spikes momentarily arise, which adversely influences the coating quality. The or each voltage level is generated from an externally supplied AC voltage by means of the AC voltage being rectified and smoothed by means of the supply system and circuit components. In this case, low-pulse circuits are used, inter alia, for cost reasons, which circuits, in comparison with higher-pulse circuits, are significantly more cost-effective and require a comparably low control outlay, but produce a higher proportion of reactive power in the external supply system, whereby the supply system is burdened and the operating costs of the coating facility are increased. Moreover, in order to be able to prevent a production outage caused by a failure of a power unit, generally at least one reserve unit is installed, which is linked to the external supply system via further supply system and circuit components. However, by virtue of such redundant components, which as a rule are not utilized, the costs for the coating facility are furthermore increased.

SUMMARY

Consequently, according to various embodiments, a power control device for an electrochemical coating facility can be specified which is as cost-effective as possible and which ensures that the coating facility is operated as efficiently and reliably as possible, while at the same time ensuring a high coating quality.

According to various embodiments, a power control device of a power supply system—which comprises a number of anodes and a number of cathodes—of an electrochemical coating facility is specified, having a plurality of control modules, wherein each control module is designed to form and control a local current flow having a predetermined magnitude as a function of the location and as a function of the time between an anode and a cathode of the power supply system.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of a power control device of a power supply system of an electrochemical coating facility is explained below with reference to a drawing, in which:

FIG. 1 shows a circuit diagram of the power control device in schematic illustration, and

FIG. 2 shows a further circuit diagram of the power control device in schematic illustration.

Mutually corresponding parts in different figures are provided with the same reference symbols.

DETAILED DESCRIPTION

A conventional dipping bath coating facility for vehicle bodies usually comprises two to four power units. The first unit feeds the dipping bath with a predetermined DC voltage. The current intensity in the dipping bath and thus the layer application per unit time decrease in the course of the coating process. Starting from a specific point in time, the second power unit is connected in, which feeds the bath with an increased voltage, with the result that the layer application per unit time increases again and thereby corresponds, on average over time, to a predetermined (constant) value. The currents and voltages present in the bath are subject to global boundary conditions which limit a continuous controllability of the electric potential present at the dipping bath and thus define the (above-mentioned) voltage levels. Consequently, threshold values exist, for example, which are to be achieved in order to ensure an effective layer application; on the other hand, limit values also exist which must not be exceeded in order that no partial discharges arise at the object to be coated. What is relevant for ensuring a high coating quality, however, is a layer application per unit time which is as constant as possible for all points in time. However, the spatial distribution of the layer thickness over the surface of the coated object is controllable only to a limited extent, which is disadvantageous particularly when a spatially variable layer application is intended to be effected in a targeted manner.

Since both power units are fed by an AC voltage from an external supply system, a power converter circuit with rectifiers is set up, which converts the AC voltage/AC current into a pulsed DC voltage/pulsed DC current. Buffer capacitances and inductances are used to smooth the voltage and the current by reduction and compensation of the fluctuation amplitudes.

On the supply system side, boundary conditions imposed on the AC voltages and currents with regard to the amplitudes thereof and the relative phase shifts thereof correspond to the boundary conditions for the currents and voltages in the dipping bath. In particular, specific minima for the phase shift between AC voltage and AC current in the power converter circuit are predetermined by the voltage levels in the dipping bath. As a result, however, a so-called displacement reactive power is produced in the supply system, which, in correspondence to the minima of the phase shift, is reducible in each case only to a specific value.

Furthermore, for cost reasons, low-pulse power converter circuits are usually used, which, under load in the high-frequency harmonic wave components of the AC voltages and currents, the so-called harmonics, overall produce larger amplitude contributions in the supply system than higher-pulse power converter circuits. These harmonic contributions produce an additional reactive power within the supply system.

The various embodiments are based on the consideration, therefore, of modularizing the power control for the bath-feeding power supply system. A significant proportion of the inadequacies with regard to the realizability of a uniform layer application onto an object to be coated and also the irreducibility of the reactive power which arises in the course of the bath power supply in the external supply system can be attributed to the fact that a high global bath current is produced with the aid of a small number of power units. The number of variable parameters is correspondingly small given defined boundary conditions. In contrast thereto, with the aid of a higher number of decoupled and separately drivable power and control modules, in each case local currents can be formed between a respective cathode and a respective anode in the bath and be controlled. It is thereby possible, in particular, to realize layer applications of different heights per unit time by means of targeted driving in defined spatial zones of the bath; by way of example, it is thereby possible, in the case of a vehicle body, for the B-pillar to be coated to a greater extent than the vehicle roof in the course of a coating process. Equalization effects in the bath which endeavor to homogenize a spatially and temporally defined distribution of the flow field in the bath can be avoided or reduced by means of circuitry measures and by means of a suitable circuit network topology.

The currents controlled by the individual control modules are not subject to the same boundary conditions as a global bath current, with the result that, in particular, the amplitudes and the relative phase shifts of and respectively between the AC voltages and currents present at the control modules on the supply system side can be smaller. As a result, the displacement reactive power in the supply system is reduced overall. Furthermore, the harmonic components of the AC voltages and currents which are caused by the individual control modules which are decoupled from one another are statistically independent of one another, such that, as a result of a statistical interference of the waves, the amplitude of the total reactive power in the supply system which can be attributed to harmonic effects is significantly reduced.

Furthermore, by virtue of a relatively large number of control modules, there is no need for any additional number of modules as reserve units. The system is already highly redundant, and the failure of a control module during a coating process therefore does not lead to significant impairment of the process. Furthermore, it is possible for individual control modules or module groups to be selectively switched on or off in the course of a coating formulation.

The consideration discussed above, on which the various embodiments are based, can be applied to the generalized case in which the dipping bath is substituted by a medium of an electrochemical coating facility of general type.

In one embodiment of the power control device, a or each control module comprises a circuit arrangement having a number of power converters, in particular having a number of rectifiers. By means of a rectifier, an AC current from the external supply system is converted to a DC current with which the dipping bath is fed. By means of a united and iteratively extendable circuit arrangement comprising a plurality of rectifiers, in particular in a parallel circuit, a high pulse number can be achieved, whereby the proportion of harmonics of such a circuit is iteratively reduced in a corresponding manner. The pulse number describes the number of coperiodic voltage or current waves which are triggered within a wave period, and the relative phase shift between two successive partial waves is given here by the period duration divided by the pulse number.

Preferably, a or each power converter, in particular a or each rectifier, is connected to a number of anodes or cathodes of the power supply system. In the case of an electrolytic coating facility for coating with metals, the object that is respectively to be coated is conductively connected to a cathode and the coupling of the or each power converter is preferably effected anodally.

In an advantageous manner, a or each circuit arrangement of power converters is realized as a series circuit formed by a controlled rectifier and an uncontrolled rectifier.

This local circuit topology is based on the principle of a so-called boost and buck connection, by means of which the phase shift between AC voltage and current is optimized on the supply system side during load operation, and a correspondingly low displacement reactive power is thus realized.

In one expedient development of the circuit arrangement of power converters, a or each controlled rectifier is realized as a thyristor bridge and/or a or each uncontrolled rectifier is realized as a diode bridge. Such a combination has the advantage that an uncontrolled diode bridge is significantly more cost-effective than a controlled power converter.

Preferably, the power control device is designed to the effect that a or each control module is connected to a number of isolation transformers.

In a further embodiment of the power control device, a or each controlled rectifier is connected to a respective isolation transformer and a or each diode bridge is connected to a respective further isolation transformer.

Such an embodiment is realized for example in the case of a circuit arrangement in which a diode bridge and a controlled rectifier which are connected in series with one another are connected to a first and second isolation transformer, respectively. The first isolation transformer feeds the diode bridge with a first current, which is in phase with the external AC voltage, and the second isolation transformer feeds the controlled rectifier with a second current, which is phase-shifted by 30 degrees with respect to the first current on the supply system side. Such a 12-pulse feeding, that is to say a feeding that is phase-offset by 30 degrees in each case, of this rectifier circuit can be realized for example with a first isolation transformer of vector type Dy0 and with a second isolation transformer of vector type Dy5 from a 6-pulse energy feed in which the phase shifts are 60 degrees in each case. A relatively cost-effective 12-pulse rectifier circuit is thus presented, which is advantageous with respect to a low-pulse rectifier circuit, in particular with respect to a 6-pulse rectifier circuit, with regard to the total harmonic generation.

If, by contrast, a 12-pulse feeding is already available on the supply system side, then the isolation transformers can be embodied identically in respect of their type, for example as isolation transformers of vector type Dy0.

In addition to the total reduction of the harmonic components which result from the statistical independence and thus from the statistically uniformly distributed interference of the harmonic fluctuations which are triggered by the individual control modules, an additional reduction of the harmonics that have a perturbing effect on the external supply system is achieved by virtue of the fact that their amplitudes scale inversely proportionally to the number of control modules.

Consequently, a particularly effective reactive power reduction is achieved overall with the displacement reactive power reduction. Consequently, the power factor of the power control device, which describes the ratio of the effectively utilized active power to the total power in the supply system including the reactive power, can attain for example a value of more than 0.94, and, in the case of a rated load of 12.5%, even more than 0.8. The reactive power reduction leads, in particular, to relief of the loading of the feeding supply system transformers.

By virtue of a 12-pulse rectifier circuit, on the DC side twice as many current and voltage maxima as in the case of 6-pulse driving arise. The amplitudes of the maxima are likewise smaller. Consequently, the DC current produced by the 12-pulse rectifier circuit and the DC voltage have comparatively small fluctuations. In the case of a suitable 12-pulse rectifier circuit, the fluctuation amplitude can be less than 1% of the DC current intensity or DC voltage produced. The required buffer capacitances and inductances for DC voltage smoothing and for DC current smoothing, respectively, which are realized for example by a smoothing capacitor and a DC inductor, respectively, can therefore advantageously be made smaller than in the case of a low-pulse rectifier circuit, with the result that overall the efficiency and the economic viability of the power control device are increased.

In a further embodiment variant of the power control device, a decoupling circuit is formed between a or each control module and in each case a number of anodes, which decoupling circuit decouples the respective control module from the dipping bath. The decoupling circuit comprises a plurality of diodes connected in series, which diodes are in each case connected in the forward direction to a respective anode.

A return-flow compensation is realized with the aid of such a decoupling circuit with the result that a flow field set in a defined manner in the bath between the anodes and cathodes does not collapse and/or is homogenized. Equalizing currents between locally adjacent feed locations are prevented.

Preferably, the decoupling circuit comprises a series circuit formed by a first diode and a second diode, which is in each case connected in the forward direction to a first anode and/or to a second anode, wherein the first diode is connected in the reverse direction to the control module via a switching element and an inductance, and wherein the second diode is connected in the reverse direction to the first diode, to the first anode and to a smoothing capacitor.

The current for both anodes flows via the first diode, and only the current for the second anode flows via the second diode. The voltage drop respectively at the first and at the second diode can lead to different voltages at the first and at the second anode. This voltage difference is compensated for by the fact that the cable path for an anode via the bath is inevitably longer, however.

The provision of a smoothing capacitor between the two series-connected diodes of the decoupling circuit has the advantage that no resonant circuit can arise between the smoothing capacitor and the inductance required for smoothing the total current flow, said inductance being provided as a DC inductor coil, in particular. The smoothing capacitor is charged by the power converter circuit via the inductance and also via the first diode in the forward direction. Return oscillation of the energy from the smoothing capacitor to the diode is not possible, however, as a result of the blocking effect of the diode. Therefore, the energy can only discharge via the resistance of the bath. In particular, undesirable equalizing discharge and compensation processes within the bath are thus prevented. Furthermore, damping that would be required for a resonant circuit, and exhibits loss of energy, is not required.

A computing unit with a simulation model for simulating voltage and/or current predeterminations is expediently set up for the power control device. Such a simulation calculates, in particular, voltage and current predeterminations in the bath and the parameters dependent thereon for a coating process.

The definition of location-dependent desired coating thicknesses in a CAD representation of the object to be coated is produced by an operating program used for predetermining the voltage and/or the current that is output to the bath via an or each anode, as a function of the object position in the bath.

FIG. 1 schematically illustrates a circuit diagram of a power control device 1 of a power supply system 2 of an electrochemical coating facility.

The potential matching indicated by the coating process takes account of the matching of the secondary voltage of the isolation transformers. It brings about an additional optimization of the reactive power component in the driving supply system.

The power supply system 2 comprises a plurality of cathodes 3, which are conductively connected to a number of objects 4 to be coated, and also a plurality of anodes 5 respectively grouped in pairs. The cathodes 3 with the objects 4 to be coated and the anodes 5 are introduced into a dipping bath containing a metal salt solution.

The power control device 1 comprises a number of control modules 6 each having a series circuit 7 formed by a controlled thyristor bridge 8 and an uncontrolled diode bridge 9. Both the thyristor bridge 8 and the diode bridge 9 are connected to a respective three-phase isolation transformer 10 and 11 on the supply system side. The thyristor bridge 8 is connected in the forward direction to a pair of anodes 5 via a decoupling circuit 12. The decoupling circuit 12 comprises a first and a second diode 13 and 14, respectively, which are in each case connected in the forward direction to an anode 5 of the pair of anodes 5. The first diode 13 is connected in the reverse direction to the thyristor bridge 8 via a switching element 15 and a DC inductor 16, and the second diode 14 is connected in the reverse direction to the first diode 13 and to the anode 5 connected thereto in the forward direction, and also to a smoothing capacitor 17.

The isolation transformers 10 and 11 feed the thyristor bridge 8 and the diode bridge 9, respectively, with an AC voltage in each case, wherein the AC voltages are in phase, or have a phase angle of 30 degrees with respect to one another. The series circuit 7 formed by thyristor bridge 8 and diode bridge produces therefrom and from the AC current of identical frequency that arrives via the isolation transformers 10 and 11 a pulsed DC voltage and a pulsed DC current, respectively, the fluctuation amplitudes of which are smoothed with the aid of the smoothing capacitor 17 and by means of the DC inductor 16, respectively. In this case, the formation of an LC resonant circuit from the DC inductor 16 and the smoothing capacitor 17 is prevented by the first diode 14 of the decoupling circuit 12, said first diode being arranged in intervening fashion in terms of circuit technology, since the energy stored in the electric field of the smoothing capacitor 17 cannot flow back as a current in the reverse direction of the first diode 14 to the DC inductor 16. Equalizing effects of the fields between the cathodes 3 and the anodes 5 are avoided by means of the decoupling circuit 12.

FIG. 2 shows a further circuit diagram of the power control device shown in FIG. 1 according to FIG. 1 in schematic illustration.

What can be seen are the control modules 6 having the thyristor bridges 8 and the diode bridges 9, which are connected to the power supply system 2 by means of the isolation transformers 10 and 11, respectively, and which are connected to the anodes 5 on the bath side. In contrast to FIG. 1, the pairs of anodes 5 illustrated therein, which are connected to a respective control module 6, are depicted schematically as a unit in this illustration. The decoupling circuits 12 illustrated in FIG. 1 are not illustrated here. The region of the dipping bath 18 is identified by a separating line 19.

DC voltages and DC current flows of different magnitudes are respectively present at the anodes 5 depending on their linear position with respect to the dipping bath 18, in order to obtain a uniform layer application on a vehicle body that is guided past the anodes 5 in the dipping bath 18. The thyristor bridges 8 and the diode bridges 9 of the control modules 6 which are connected to the respective anodes 5 produce these respective DC voltages and DC currents from AC voltages and AC currents, respectively, which are provided by the isolation transformers 10 and 11, respectively, with the amplitude respectively required. Depending on the position with regard to the dipping bath 18, therefore, the isolation transformers 10 and 11 are respectively designed for transforming voltage differences of different magnitudes.

The further details of the illustration correspond to the details of the illustration in FIG. 1 and can be gathered from FIG. 1.

The control modules can be connected in parallel in any desired number for the purpose of increasing current, wherein the interconnection can be embodied according to the master-slave principle, in particular. Conventional systems of ADC (anodic dip coating) and CDC (cathodic dip coating) embodiment can thus be simulated identically. Mixed operation of ADC and CDC is not ruled out.

The DC circuit is composed, in particular, of the series circuit formed by unregulated and regulated power converters, and also of storage elements (L and C). The application also covers any desired order of these elements in the series circuit. By way of example, the order of controlled bridge, inductance, uncontrolled bridge, capacitive smoothing, diode is conceivable.

In order to further reduce the 12-pulse supply system perturbations, in particular two systems can be connected in series, the isolation transformers of which are offset by an angle of 15 degrees with respect to the first system. 

1-10. (canceled)
 11. A power control device of a power supply system with a number of anodes and a number of cathodes of an electrochemical coating facility, comprising: a plurality of control modules which can be driven independently of one another, wherein each control module is designed to form and control a local current flow having a predetermined magnitude as a function of the location, wherein layer applications of different heights per unit time can be realized on an object to be coated by means of targeted driving in defined spatial zones of the bath, and as a function of the time between an anode and a cathode of the power supply system.
 12. The power control device according to claim 10, wherein a or each control module comprises a circuit arrangement having a number of power converters.
 13. The power control device according to claim 12, wherein a or each power converter is connected to a number of anodes or cathodes of the power supply system.
 14. The power control device according to claim 12, wherein a or each circuit arrangement of power converters is realized as a series circuit formed by a controlled rectifier and an uncontrolled rectifier.
 15. The power control device according to claim 14, wherein at least one of: a or each controlled rectifier is realized as a thyristor bridge, and a or each uncontrolled rectifier is realized as a diode bridge.
 16. The power control device according to claim 12, wherein a or each control module is connected to a number of isolation transformers.
 17. The power control device according to claim 15, wherein a or each controlled rectifier is connected to a respective isolation transformer and a or each diode bridge is connected to a respective further isolation transformer.
 18. The power control device according to claim 10, wherein a decoupling circuit, comprising a plurality of diodes connected in series, which diodes are in each case connected in the forward direction to a respective anode, is formed between a or each control module and in each case a number of anodes.
 19. The power control device according to claim 18, wherein the decoupling circuit comprises a series circuit formed by a first diode and a second diode, which is in each case connected in the forward direction to a first anode and/or to a second anode, wherein the first diode is connected in the reverse direction to the control module via a switching element and an inductance, and wherein the second diode is connected in the reverse direction to the first diode, to the first anode and to a smoothing capacitor.
 20. The power control device according to claim 19, wherein a computing unit with a simulation model for simulating at least one of voltage and current predeterminations is provided.
 21. A method for controlling of a power supply system with a number of anodes and a number of cathodes of an electrochemical coating facility, comprising the step of driving a plurality of control modules independently of one another, wherein each control module is designed to form and control a local current flow having a predetermined magnitude as a function of the location, wherein layer applications of different heights per unit time are realized on an object to be coated by means of targeted driving in defined spatial zones of the bath, and as a function of the time between an anode and a cathode of the power supply system.
 22. The method according to claim 21, further comprising the step of providing a or each control module with a circuit arrangement having a number of power converters.
 23. The method according to claim 22, further comprising the step of connecting a or each power converter to a number of anodes or cathodes of the power supply system.
 24. The method according to claim 22, wherein a or each circuit arrangement of power converters is realized as a series circuit formed by a controlled rectifier and an uncontrolled rectifier.
 25. The method according to claim 24, wherein at least one of: a or each controlled rectifier is realized as a thyristor bridge, and a or each uncontrolled rectifier is realized as a diode bridge.
 26. The method according to claim 22, further comprising the step of connecting a or each control module to a number of isolation transformers.
 27. The method according to claim 25, further comprising the step of connecting a or each controlled rectifier to a respective isolation transformer and connecting a or each diode bridge to a respective further isolation transformer.
 28. The method according to claim 21, further comprising the step of forming a decoupling circuit, comprising a plurality of diodes connected in series, which diodes are in each case connected in the forward direction to a respective anode, between a or each control module and in each case a number of anodes.
 29. The method according to claim 28, further comprising the step of providing the decoupling circuit with a series circuit formed by a first diode and a second diode, which is in each case connected in the forward direction to a first anode and/or to a second anode, wherein the first diode is connected in the reverse direction to the control module via a switching element and an inductance, and wherein the second diode is connected in the reverse direction to the first diode, to the first anode and to a smoothing capacitor.
 30. The method according to claim 29, further comprising the step of providing a computing unit with a simulation model for simulating at least one of voltage and current predeterminations. 